Electric Field Energy Density: Plates of Q & A

AI Thread Summary
The discussion revolves around calculating the energy density of an electric field between two charged non-conducting plates. The user is uncertain about their calculations and questions the accuracy of their teacher's provided answer of 1.7E+7. They express confusion over the lack of information regarding the distance between the plates, which is crucial for solving the problem. Other participants suggest that the user may have computed the energy density correctly, arriving at an answer of 5.085E+5 J/m³. The user highlights the challenge of relying on forum assistance due to limited guidance from their professor.
justin15501
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Homework Statement


Two large non-conducting plates of surface area A=.025m^2 carry equal but opposite charges Q = 75microC. What is the energy density of the electric field between the two plates.

Homework Equations


I wrote the equations on my attempt. This was a multiple choice problem and the correct answer he gave us was 1.7*10^7 but he's been wrong before.

The Attempt at a Solution


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Any help is appreciated!
 
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Try again. A=.025 The rest looks correct.
 
Charles Link said:
Try again. A=.025 The rest looks correct.
The new answer I get is 508475 instead of 5085. I feel like I may be using a wrong equation or something? My teacher didn't really lecture on energy density at all so I'm a bit uncertain.
 
justin15501 said:
The new answer I get is 508475 instead of 5085. I feel like I may be using a wrong equation or something? My teacher didn't really lecture on energy density at all so I'm a bit uncertain.
I think you got the rest correct. I think your teacher's answer of 1.7E+7 may be incorrect.
 
Last edited:
Charles Link said:
I think you got the rest correct. I think your teacher's answer of 1.7E+7 may be incorrect.
My answer isn't even one of the multiple choices?
 
What is the distance between plates?
 
Energy density is
ω = W / V = E / (Ad)
where W - energy, V - volume, d - distance between the plates.

Capacity of the system (actually, we consider a capacitor)
C = ε0A/d

Energy
W = Q2/2С

Hope, it is enough to find the solution.
 
Irene Kaminkowa said:
What is the distance between plates?
Irene Kaminkowa said:
Energy density is
ω = W / V = E / (Ad)
where W - energy, V - volume, d - distance between the plates.

Capacity of the system (actually, we consider a capacitor)
C = ε0A/d

Energy
W = Q2/2С

Hope, it is enough to find the solution.
The problem doesn't say the distance between the plates.
 
justin15501 said:
The problem doesn't say the distance between the plates.
But have it in your mind.
Derive the final formula.
 
  • #10
@justin15501 Please let us know if your teacher agrees with your answer. I do think you computed it correctly.
 
  • #11
My answer is
5.085⋅105 J/m3?

Regards to your teacher )
 
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  • #12
Charles Link said:
@justin15501 Please let us know if your teacher agrees with your answer. I do think you computed it correctly.
Irene Kaminkowa said:
My answer is
5.085⋅105 J/m3?

Regards to your teacher )
Well the way it works is my professor goes over this packet of problems (including this one) the day before the exam and leaves us to figure out the questions we got wrong on our own. And there's 165 questions in this packet. It's not a very ideal situation. If I get something wrong, it's not like I can go to him for help. I'm left to rely on the kind folks of this forum to help me however they can!
 

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